Charlie Mullins, left, was a business adviser to David Cameron and George Osborne, and has often been photographed with government ministers.
Photograph: Pimlico Plumbers

A prominent Conservative supporter is backing a legal challenge to Theresa May’s plan to trigger the two-year process of negotiating the UK’s withdrawal from the European Union without a vote in parliament.

Charlie Mullins, the head of the company Pimlico Plumbers, said he is one of the businesspeople funding a high court case that will question the government’s legal advice that article 50 can be invoked under the royal prerogative, which does not require parliamentary approval.

It follows May’s speech to the Conservative party conference in which she accused those seeking a judicial review into the government’s authority of being anti-democratic.

May told delegates in Birmingham: “Those people who argue that article 50 can only be triggered after agreement in both houses of parliament are not standing up for democracy; they’re trying to subvert it. They’re not trying to get Brexit right, they’re trying to kill it by delaying it. They are insulting the intelligence of the British people.”

Mullins, whose firm has given more than £36,000 to the Tories, told the Guardian he is helping to fund challenge because he wants the clause that triggers the start of the negotiation to only be invoked after a vote in parliament.

He has spoken to his lawyers, Mishcon de Reya, about the challenge and has met Gina Miller, the founder of the investment management group SCM Private, to discuss the challenge, he said. Miller is one of the claimants leading the high court challenge.

“We are putting our dossier together to say that the people who should have the vote is parliament,” he said. “I believe that if it goes to parliament, people are going to see common sense on it. We are the ones who are putting our heads above the parapet. There are a lot of other people involved but they would rather not be known at this stage.”

Mullins and George Osborne at the Pimlico Plumbers offices in Lambeth. Photograph: Pimlico Plumbers

The article 50 litigation began in July following the UK’s vote to leave the EU. The action is being led by claimants Miller and hairdresser Deir Dos Santos, represented by Mishcon de Reya and Edwin Coe respectively, and is set to be heard in the high court on October 13 and 17.

Mullins, who campaigned against Brexit during the referendum, said he expected to see a downturn in the economy if UK leaves the EU. The businessman, whose firm controls 200 plumbing vans across the capital, was a business adviser to David Cameron and George Osborne and has often been photographed with government ministers including Priti Patel at the Conservative party’s annual Black and White Ball. He received an OBE in 2014 for services to business.

“We are faced with a big situation where we know what is going to happen in March. I think we are going to see a rocky road out there, I think it’s going to affect the economy and we are going to see job losses. If they put controls on migrants coming in and out then it’s certainly going to affect the building industry and construction industry.”

In her speech, the prime minister also confirmed that the attorney general, Jeremy Wright QC, would be resisting the legal challenge in the high court, alongside James Eadie QC, Jason Coppel QC, Tom Cross, and Christopher Knight, who have all been named as counsel for the government.

After the speech, Mark Elliott, professor of public law at the University of Cambridge, tweeted that May appeared to have “weaponised” the attorney general “in the service of democracy”.